Fix a case where exptidx wasn't being initialized in Slice._load. More

importantly, fix several places where inserts were using %d as argument
placeholders in queries. When using the MySQLdb module in python, you
can only use %s as a placeholder in "prepared" queries---the module looks
at the types of the values to see how to format them in the query. This
behavior is insidious and contrary to the python standard DB interface
documentation, but that's how it is.